Keep the racket down, I’m trying to find a mate! That could
be the plea of nocturnal Chinese tree frogs, which have developed unique,
high-frequency vocal skills to make themselves heard by potential mates in
their noisy habitat.

The frogs' sounds are no mere "ribbits," but ultrasound
squeaks designed to be heard over the rushing of a nearby mountain river that
roars in the audible sound range.

"The background noise covers the entire human hearing
range," said Peter Narins, a biologist at the University of California,
Los Angeles. "Just talking to my colleagues there, you really have to
raise your voice to be heard. We think these animals are under evolutionary
pressure to avoid that channel and go above it."

The ultrasonic creatures, known as torrent frogs (Odorrana tormota), live near China's Yellow
Mountain. They are tiny brown- and black-striped critters, less than one and a
half inches (35 mm) across. Whereas most frogs chirp in the audible sound
range, this species is one of only a few that produce higher-frequency ultrasound
noises.

The noises are mainly courtship calls. Males cry out to
advertise their virility, and females squeak just before they ovulate to
announce their fertility.

Here, froggy froggy!

Narins and his colleagues, led by Jun-Xian Shen of the
Chinese Academy of Sciences in Beijing, recorded the squeaks of female Odorrana frogs using an ultrasonic
microphone. When the researchers played back the females' sounds, male frogs
would call out and hop toward the speakers.

The scientists found that the eager male frogs could
pinpoint where the females' noises were coming from with an extraordinary
degree of precision.

"From a meter away you can play the sound of a female,
and in some cases, in one leap the males jump right to the center of the
sound," Narins told LiveScience.
"Their precision was remarkable — it rivals that of the best vertebrate
performers, such as barn owls, dolphins and humans."

Male Odorrana frogs
can locate the source of a female's mating call to within about one degree on
average. In contrast, most other amphibians
can only pinpoint sound to within 16 to 23 degrees, Narins said.

Sound shadow

The key to these frogs' amazing abilities lies in the fact
that they produce their calls in ultrasound, which has shorter wavelengths than
audible sound.

Animals compare the intensity of the sound signals received
by their two ears to localize where a sound is coming from. For this reason,
the size ratio of the animal's head compared to the wavelength of the sound is
important: If the sound waves are much smaller than the head, many waves get
bounced off and the signal coming to one ear is much stronger than to the
other. This large differential, called a sound shadow, helps the animal locate
the noise's origin.

However, if the sound waves are larger than the head, they
can reach the ear on the other side without getting bounced off, so the two
ears receive comparable signals, making it difficult to distinguish where the
noise is coming from.

Working with Nature

By chirping in frequencies above the audible din of nature,
the frogs have found an ingenious solution to their environmental predicament.

"If you have a really noisy environment you have to
figure out how to overcome it," Narins said. "That's how nature
works. You have to be adapted as well as possible to your environment. They
figured out how to do it very well."

The researchers detailed their findings in the May 11 issue
of the journal Nature. The study was
funded by the National Natural Sciences Foundation of China, U.S. National
Institute on Deafness and Other Communication Disorders, UCLA Academic Senate, Paul
S. Veneklasen Research Foundation, and U.S. National Science Foundation.

Clara Moskowitz

Clara has a bachelor's degree in astronomy and physics from Wesleyan University, and a graduate certificate in science writing from the University of California, Santa Cruz. She has written for both Space.com and Live Science.